<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			
			*{
				margin: 0;
				padding: 0;
			}
			.father{
				width: 200px;
		     	height: 200px;
				border: 1px solid #000;
				margin: 100px auto;
			
			/* border-radius: 100px 100px 100px  100px; */
			/* border-radius: 100px 0 0 0; */
			border-radius: 50%;
			}
			
			.son{
				height: 100px;
				width: 100px;
				background: rgba(255, 0, 0, 0.3);
			}
		</style>
	</head>
	<body>
		
		<div class="father">
			<div class="son"></div>
		</div>
		
	</body>
</html>